Silicon Controlled Rectifiers (SCRs) are popular as electrostatic discharge (ESD) protection devices because of their capacity for handling large ESD currents while needing only a small amount of silicon area. However, their main drawback is that, once activated, the voltage across the SCR needs to be decreased below its “hold voltage” to make the SCR shut down. For short-base SCR's this hold voltage may lie in the range of 2-3 V. If such an SCR is implemented in a product that operates in the 3-5 V range, the device is prone to latch-up, which generally leads to device failure.
A long-base SCR is an SCR engineered to have its holding voltage above the intended operating voltage of a product. The high holding voltage is achieved by placing the NPN emitter and PNP emitter of the device further apart. A disadvantage of a device of this kind is that its operating voltage during an ESD event is typically higher than for a short-base SCR, and this may be harmful to the integrated circuit (IC) that is being protected. If an ESD device of this type is used, the rest of the IC needs to be carefully engineered to make sure it can handle the higher operating voltage. This process is time consuming, sometimes requires special software tools, and can introduce ESD-failures if despite all precautions a weak spot is overlooked. Another disadvantage is that long-base SCR's can may only be used on low-ohmic substrate wafers. For IC's in high-voltage processes, or RF-processes where typically high-ohmic substrate wafers are used, long-base SCR's usually have a holding voltage well below 5V, and accordingly cannot be used as protection devices for products operating at 5V.